This disclosure relates to aircraft boundary-layer flow-control systems, and more specifically, to methods and apparatus for encouraging laminar flow along the surface of an airfoil or body.
Laminar flow along a surface of an airfoil is typically achieved by reducing the magnitude of disturbances and instabilities in the boundary-layer. By keeping these fluctuations small, the nonlinear interactions leading to turbulence can be curtailed and/or delayed. Currently, the most robust methods for controlling the disturbance amplitudes are based on modifying the boundary-layer mean flow via airfoil geometry (i.e., by tailoring the pressure gradient) or by applying surface suction. Since modifications to the pressure gradient do not actively consume power, this approach has been termed “natural laminar flow”. The successful application of this approach and attainment of drag reduction benefits has been demonstrated both theoretically and in testing for nominally two-dimensional boundary layers.
Surface discontinuities may disrupt the laminar boundary layer of air over an airfoil (e.g., an aircraft wing) and cause it to become turbulent. A turbulent boundary layer may be characterized by increased mixing between layers of air within the boundary layer. The drag caused by a turbulent boundary layer may be as much as 5 to 10 times greater than the drag of a laminar boundary layer. This transition from laminar flow to turbulent flow within the boundary layer may increase drag. Also, this transition may produce undesirable noise, decrease fuel efficiency, and/or other undesirable effects during flight. These discontinuities may be caused by, for example, without limitation, steps and/or gaps in joints between wing skin panels and/or steps and/or waviness due to fasteners that extend through the skin panels and fasten the panels to the wingbox.
It is well known that systems incorporated in an airfoil for other purposes may contribute to turbulent flow. For example, anti-icing systems are widely used for the prevention of ice buildup on leading edges of aircraft structures. It is known to install a bleed-air wing anti-icing or de-icing system near the leading edge of a wing. The incorporation of an anti-icing system in a wing leading edge may adversely affect airflow in the boundary layer.
In addition, modern aircraft may use a variety of high-lift leading and trailing edge devices to improve high-angle of attack performance during various phases of flight, including takeoff and landing. Existing leading edge devices include leading edge slats and Krueger flaps. Krueger flaps have generally the same function as leading edge slats, but rather than retracting aft to form the leading edge of the cruise wing, Krueger flaps are hinged, and typically fold into the lower surface of the wing when stowed. When deployed, Krueger flaps extend forward from the under surface of the wing, increasing the wing camber and maximum coefficient of lift. In the case of a typical Krueger flap, a slot or gap is created between the flap and the wing as the flap is extended forward. During certain operating conditions, air can flow through this slot to energize the airflow over the upper surface of the wing, and improve overall airflow characteristics over the wing. A two-position, high-height, variable-camber Krueger flap assembly is disclosed in U.S. patent application Ser. No. 13/867,562, the disclosure of which is incorporated by reference herein in its entirety.
A wing leading edge structure that encourages laminar flow, integrates a bleed-air anti-icing system, and further integrates a two-position, high-height, variable-camber Krueger flap would be useful.